Predictive value of foetal superior temporal sulcus asymmetry for neonatal speech discrimination

While some anatomical brain asymmetries are seen across primates, the earlier appearance and larger depth of the right superior temporal sulcus (STS) is specific to human foetuses. Interestingly, the degree of STS asymmetry varies between foetuses, and it has been shown that this interindividual variability is related to the functional lateralization of the language network in school-aged children. It remains unclear, however, whether it is also indicative of language localization and functioning shortly after birth. In the present longitudinal study, we prospectively examined the predictive value of foetal STS asymmetry for neonates' language lateralization and neural speech discrimination. We measured the STS depths and volumes in neurotypical foetuses (N = 35) using foetal MRI. After birth, we investigated the neonates' haemodynamic response to forward and backward speech using functional near-infrared spectroscopy. We hypothesized that less rightward asymmetry of the STS depths in the foetal brain is related to increased left language lateralization and to a greater haemodynamic difference between speech conditions in the left hemisphere in neonates. While the foetuses demonstrated an overall rightward asymmetry of the STS depths and volumes, the degree of asymmetry varied between individuals. After birth, the group activated left frontal and right temporal regions during the speech discrimination paradigm. Again, there was variability in the degree of neural activation in response to speech. Importantly, we found that children with a foetal STS depth asymmetry towards the left hemisphere activated their right hemisphere less for forward speech (r = -0.58, P = 0.002) and differentiated less between forward and backward speech in their right hemisphere (r = -0.48, P = 0.014). Contrary to our initial hypothesis, the results suggest that an earlier structural development of the left temporal lobe goes along with reduced involvement of the right hemisphere, rather than an increased involvement of the left hemisphere, during neural speech discrimination. Given that rightward asymmetry of language-related brain areas has been associated with weaker language abilities, the present study provides important preliminary data regarding the neural underpinnings of language development during the prenatal and early postnatal period.